Velocity modulation electron discharge devices



July 3, 1962 N. c. BARFORD 3,042,831

VELOCITY MODULATION ELECTRON DISCHARGE DEVICES Filed Nov. 13, 1946 r ly A I an).

IN V EN TOR. Narman 6". Baribrd A 7' TOR/V5 Y United States Patent 1 3,042,831 VELOCITY MODULATION ELECTRON DISCHARGE DEVICES Norman Charles Barford, Prittlewell, Southend, England, assignor to Electric & Musical Industries Limited,

Hayes, England, a company of Great Britain Filed Nov. 13, 1946, Ser. No. 709,665 In Great Britain Mar. 22, 1945 Section 1, Public Law 690, Aug. 8, 1946 Patent expires Mar. 22, 1965 25 Claims. (Cl. 315-35) This invention relates to velocity modulation electron discharge devices. which an electron beam is caused to pass through a hollow resonator where the beam becomes velocity modulated, the velocity modulated beam being then allowed to become charge density modulated and being then passed through a further hollow resonator which extracts energy from the charge density modulated beam. In such devices it is important, in order to afford a useful interchange of energy, that the transit time of the electrons in passing through the resonators is less than half a period of the oscillatory field which is set up in the resonators. This factor, therefore, sets a limit to the frequency at which it is possible to employ such devices and when it is desired to employ such devices at frequencies above 10,000 mc./s., the construction of resonators in order to maintain the required transit time becomes exceedingly difiicult.

It is, therefore, the object of the present invention to provide an improved velocity modulation electron discharge device with a view to overcoming or reducing this difficulty.

According to the invention there is provided a traveling wave electron discharge device of the velocity modulation type comprising a coiled length of hollow waveguide and means for projecting a beam of electrons through said coiled waveguide the arrangement being such that a repeated or continuous interchange of energy occurs between said electrons and the field set up in the waveguide.

In one form of the invention a series of apertures may be provided in the walls of a coiled hollow waveguide and the electron beam may be caused to pass through said apertures along a line parallel to, but spaced from the axis about which the coil is wound, whilst in another form of the invention the electron beam may be caused to pass along said axis with the wall of the waveguide adjacent the axis being provided with a helical slot, or said wall may be omitted. In the former case. the electrons in effect repeatedly traverse the field set up in the I waveguide so that, while the interchange of energy for one traversal may be quite small, the interchange of energy after several traversals may be of useful magnitude. In the latter case the electrons interact continuously with a high frequency wave which travels efiectively at the same axial velocity as the electrons so that a continuous interchange of energy can take place.

In order that the said invention may be clearly understood and readily carried into effect, it will now be more fully described with reference to the accompanying drawings, in which:

FIGURE-l is an elevation illustrating diagrammatically one form of discharge device constructed according to the invention.

FIGURE 2 is an end elevation of the coiled waveguide shown in FIGURE 1,

FIGURE 3 is a section taken along the line 3-3 of FIGURE 2,

FIGURE 4 is a view similar to FIGURE 3 but illustrating another form of the invention,

FIGURE 5 is a diagrammatic illustration of the use Many such devices are known inice of the device according to the invention as a generator of. self-maintained oscillations,

FIGURE 6 is a view similar to FIGURE 3 but illustrating another modification of the invention.

As shown in FIGURES l, 2 and 3 of the drawings, the reference numeral 1 indicates a hollow waveguide which is coiled in a helical manner. The coiled waveguide is shown in FIGURE 3 as made up of two tubular conductive members 1a and 1b contacting the inner and outer peripheries of a radially-extending fiat helical conductive strip 10. A series of apertures 2 are provided in the adjacent conducting walls of the convolutions of the coil, in the strip 1c, these apertures being arranged in line with one another and on an axis parallel to, but spaced from the axis of the coil. At one end of the coil there is provided an electron gun compris ing a cathode 3 and'a cathode shield 4 whilst at the other end of the coil there is provided a collecting electrode 5. It should be noted that a more complicated type of gun may be required to give adequate focussing. The electrons from the cathode 3 are caused to pass through the series of apertures 2 and thus through the adjacent convolutions of the coil, the electrons being finally collected by the electrode 5. The space through which the electrons pass will, of course, require to be evacuated and either the whole coil may be enclosed in an evacuated envelope 8 as shown in dash lines in FIGURES 4 and 6, or alternatively, an evacuated envelope need only be provided to enclose the beam apertures and the electrodes 3, 4 and 5. This latter construction may, however, give rise to reflections owing to the presence of the envelope in the path of the waves propagated through the guide. 'Alternatively the cathode 3 and electrode 4 and the first aperture 2 may be enclosed by an envelope 9 sealed to the wave guide at one end and similarly the last aperture 2 and the electrode 5 by an envelope 10 sealed to the other end of the wave guide 1, the guide 1 being evacuated and the terminal ends A and B of the guide suitably sealed at 11 and 12, as shown in FIG- URE 5. Any reflections which occur due to the sealed ends of the guide can be reduced over a restricted frequency range by suitable design of window at said ends. In operation of the device shown in FIGURES l, 2 and 3, a wave is propagated through the waveguide from the input terminal A to the output terminal B with the electric vector of the wave extending substantially in the direction of the axis of the coil. The electron beam passes through the adjacent convolutions of the coil substantially in the direction of the high frequency electric vector of the wave propagated through the guide so that the electrons will receive repeated retardations or accelerations whereby the beam will become velocity modulated and eventually charge density modulated and will impart energy to the high frequency field on traversing the last few turns of the coil. It is necessary that the electrons which are most retarded, should meet a similarly phased electric vector at each time they pass from one convolution to another. By suitably choosing the shape of the guide and coil and the velocity of the electrons, this condition can be met by causing the wave to travel around one convolution of the coil whilst an electron crosses from the mid-point of one convolution to the mid-point of the next convolution. The same condition can also be met by arranging for the wave to travel around one convolution plus any integral number of guide wavelengths, whilst the electrons cross from the mid-point of one convolution to the mid-point of the next convolution. It will be understood that where mention has been made of the wave travelling around the guide it is the phase velocity of the wave which is to be considered.

Patented July 3., 1962 If desired, more than one series of apertures 2 can be employed,,togetl1er with a corresponding number of electron beams.

to be of circular shape since other shapes can be employed, for example, it may be advantageous to use arcuate shaped apertures the centers of the arcs coinciding with the axis of the coiled waveguide.

FIGURE 4 of the drawings illustrates an alternative form of theinvention in which an electron beam is caused to pass along the axis or the coil and in such a construction a helical slot 6 having the same pitch as the coil is provided in the inner wall of the hollow guide 1 adjacent said axis, as shown in FIGURE 4. [The in- Her conducting wall of the guide 1 is made up of an axially-extending flat helical conducting strip 1d engaging the inner edges of the channel shaped conductor formed by the helical conducting strip. or ribbon 1c and the outer conducting member lb. A cathode 3 and cathode shield 4 are provided at one end of the guide 1 to project an electron beam axially through the guide to a collector 5 at the other end. Alternatively, the wall of the guide adjacent said axis may be omitted. In this construction'when a wave is propagated through the guide a high frequency electric field is set up across the gap provided by the slot 6, or across adjacent edges of the walls of the waveguide if the aforesaid inner wall is omitted, theelectron beam then interacting continuously with said field during its passage through the coil. In this embodiment of the invention it is necessary for the electrons to advance one convolution of the coil whilst the wave travel-s round one convolution.

The devices shown in FIGURES l, 2,3 and 4 can be employed as amplifiers, a received high frequency signal or a signal from a low power generator being fed into the end A of the hollow guide and an amplified output being obtained from the end B. The devices shown can also be employed as mixers of a plurality of high frequency waves of different frequencies, such waves being fed into a guide'at the end A and the modulated current beam detected to afford an output of an intermediate frequency. The devices can also be employed to afiord a detected output from a modulated wave. Further, the devices can be employed as generators of selfmaintained oscillations in which case some'of the output from the end B would be required to be fed back to the input; end A. This may be efiected by providing some It is not necessary for the apertures V the whole length of the coil. Since such electrons will travel progressively more slowly, it may be necessary to cause the pitch of the coil to decrease towards the Output end B or it may be necessary to alter the shape of the waveguide towards such end so that the propagated wave travels more slowly. Initially retarded electrons will then impart energy through the field throughout their whole passage, whilst those electrons which are initially accelerated and thus abstract energy eventually become out of phase with the field and so abstract less net energy than the energy which the retarded electrons impart. Although the adjacent convolutions of the guide are shown in contact or contiguous with one another in FIGURES 1-6, it may in some cases be advisable to provide some separation between the adjacent convolutions. Furthermore, although the cross-section of the Waveguide is shown as being rectangular, it will be appreciated that other cross-sections of waveguide may be employed. In addition the waveguide need not be coiled to provide a circular helix since the helix may be of other shapes. For example, the helix may be square although this construction may not be advantageous owing to the possibility of reflections occurring at the corners of the helix.

In some cases, it may be advantageous to employ an electron beam of annular form. In FIGURE 6 the apertures 2 of FIGURES 1, 2, and 3 are replaced by a helical slot 2 around the walls of the wave guide and an annular beam is produced by means of annular cathode 3 and cathode shield 4', the electrons being collected by annular electrode 5; With the arrangement shown in FIGURE 4, an annular beam could be passed through the space in the centre of the coiled waveguide.

What I claim is:

l. A traveling wave electron discharge device of the velocity modulation type comprising a helically coiled length of hollow wave guide, one end of said wave guide constituting an input terminal for introducing a high frequency wave into said coiled wave guide for propagation therethrough along the turns of the helix, means adjacent to said end 01f said guide for projecting a beam of electrons through said coiled wave guide in the direotion of the longitudinal axis thereof, the other end of 7 said coiled wave guidefconstituting an output terminal form of' external transmission line coupled between the two ends of the coil, although it is preferred to employ an arrangement for this purpose as shown in FIGURE 5. In this figure the ends A and B- of the guide 1 are coupled together through a cavity resonator 7 in a length of guide which couples the two ends A and B. the resonator 7 being arranged to pass only one frequency so as to ensure the regeneration of oscillations at the frequency. The resonator 7 can be disposed at a position well removed from the electron beam and outside the evacuated envelope of the device, thus permitting the resonator 7 to be readily tuned. Also, since the shape of the resonator 7 is not limited by considerations of electron transittime, etc., it may be chosen to give'fine, coarse, or wide band tuning, thereby enabling the frequency of the generated oscillations to be varied without alteration of the waveguide. The resonator 7 could be interchangeable with other resonators of differing frequency bands to afiord extremely wide band tuning provided, of course, that the frequency involved is not belowthe cut-oil frequency of the waveguide.

Where the devices are required for the generation of self-maintained oscillations, it may be possible to cause the generation of such oscillations merely by closing the end A of the guide.

In FIGURES 1 to 4 the pitch of the coil is shown as being constant. It may, however, be advisable to change the pitch of the convolutions since those electrons which are retarded most should be in phase with the field along for utilizing said wave, and means including a hollow Wave guide and a frequency-determining cavity resonator electrically coupling together the two ends of said coiled wave guide for transferring energy between said ends.

2. An electron discharge device of the velocity modulation type comprising a helically coiled length of hollow wave guide adapted to have a high frequency wave propagated therethrough, the Walls of said wave guide having 1 at least one series of aligned apertures arranged in spaced relation to and substantially parallel to the longitudinal axis of the helical wave guide, and means for projecting a beam of electrons through said series of apertures.

3. An electron discharge device according to claim 2 i wherein said apertures and said beam projecting means are annularfin shape and coaxial with said longitudinal axis.

4. An electron discharge device according to claim 2.

further including means for coupling together the two ends of said coiled wave guide for transferring energy between said ends.

5. An electron discharge device according to claim 4 wherein said coupling means includes a frequency-determining cavity resonator and connecting wave guide sections.

6. A traveling wave electron discharge device of the velocity modulation type comprising a helically coiled length of hollow wave guide, one end of said wave guide constituting an input terminal for introducing a high frequency wave into said coiled wave guide for propagation therethrougb along the turns of the helix, the inner Wall of said helically coiled wave guide being provided With a helical slot, means adjacent to said end of said guide for projecting a beam of electrons through said coiled wave guide along the longitudinal axis thereof, the other end of said coiled wave guide constituting an output terminal for utilizing said wave.

7. An electron discharge device comprising an elongated hollow waveguide, means forming a part of said guide for limiting the axial velocity of a wave along said guide to a value substantially less than the wave velocity in air, one end of said guide constituting an input terminal for introducing a high frequency wave into said guide for propagation therethrough and the other end of said guide constituting an output terminal for utilizing said wave, means for projecting a beam of electrons through said guide in the direction of the longitudinal axis thereof, and means including a separate transmission line electrically coupling together the two ends of said wave guide for transferring energy between said ends, said coupling means including a frequency-determining cavity resonator and connecting waveguide sections.

8. An electron tube comprising a helical wave guide formed of channel shaped conductor having substantially rectangular cross-section, means for generating an electron beam along the axis of said helical waveguide, a fiat helical conductive strip engaging the open side of said channel, said strip being formed wtih adjacent turns spaced from each other so as to effect a helical gap of pitch equal to the pitch of said helical wave guide, said gap being disposed substantially centrally of said open side of said wave guide and permitting energy radiation from said Wave guide into the region of said electron beam, and means for introducing energy to and extracting energy from said wave guide.

9. An electron tube comprising a helical Wave guide formed of channel shaped conductor having substantially rectangular cross-section, means for generating an electron beam along the axis of said helical waveguide, a fiat helical conductive strip engaging the open side of said channel, said strip being formed with adjacent turns spaced from each other so as to effect a helical gap of pitch equal to the pitch of said helical wave guide, said gap being disposed substantially centrally of said open side of said wave guide and permitting energy radiation from said Wave guide into the region of said electron beam, and means for introducing energy to and extracting energy from said wave guide, said wave guide being formed to permit inter-action of electromagnetic wave energy flowing therein with said electron beam.

10. An electron tube comprising a continuous helical wave guide formed of channel shaped conductor having substantially rectangular cross-section, means for generating an electron beam along the axis of said helical Waveguide, a flat helical conductive strip engaging the open side of said channel, said strip being formed with adjacent turns spaced from each other so as to eifect a helical gap of pitch equal to the pitch of said helical Wave guide, said gap being disposed substantially centrally of said open side of said wave guide and permitting energy radiation trom said wave guide into the region of said electron beam, and Wave energy coupling means associated with each end of said wave guide.

11. An electron tube comprising a continuous helical wave guide formed of channel shaped conductor having substantially rectangular cross-section, means for generating an electron beam along the axis of said helical Waveguide, a flat helical conductive strip engaging the open side of said channel, said strip being formed with adjacent turns spaced from each other so as to effect a helical gap of pitch equal to the pitch of said helical wave guide, said gap being disposed substantially centrally of said open side of said wave guide and permitting energy radiation from said Wave guide into the region of said electron beam, said Wave guide being formed to permit inter-action of electromagnetic wave energy flowing therein with said electron beam, and said wave guide a having energy input and output couplings displaced Ion-'- gitudinally of said path.

12. An electron tube comprising a Wave guide of substantially rectangular cross-section helically wound around said beam path, means for generating an electron beam along the axis of said helical waveguide, a flat helical conductive strip engaging the open side of said guide, said strip being formed with adjacent turns spaced from each other so as to effect a helical gap of pitch equal to the pitch of said helical wave guide, said gap being disposed substantially centrally of said open side of said wave guide and permitting energy radiation from said Wave guide into the region of said electron beam, said wave guide being adapted to permit density modulation of said electron beam in accordance with electric wave energy flowing therein, and to extract energy from a density modulated electron beam.

13. A beam traveling wave electron tube comprising in combination an evacuated envelope, an electron gun disposed at an end of said envelope for generating an electron beam of predetermined average electron velocity, a collector electrode positioned within the'opposed end of said envelope, means for directing said generated electron beam along a path axially through said envelope and impinging said beam upon said collector electrode, a helical wave guide termed of channel shaped conductor having substantially rectangular crosssection surrounding said path, a flat helical conductive strip engaging the open side of said channel, said strip being formed with adjacent turns spaced from each other so as to elfect a helical gap of pitch equal to the pitch of said helical wave guide, said gap being disposed substantially centrally of said open side of said wave guide and permitting energy radiation from said wave guide into the region of said electron beam, and means for introducing and extracting energy from said wave guide.

14. An electron tube comprising a helical waveguide of substantially rectangular cross-section having one of its conductive bounding surfaces provided with a gap running lengthwise thereof and following the helical convolutions of said surface, means V-for'generating an electron beam along the axis of said helical waveguide, said gap being disposed substantially centrally of said one bounding surface and permitting energy radiation from said Waveguide into the region of said electron beam, and means for introducing energy to and extracting energy from said waveguide, the axial dimension of said gap being less than the axial dimension of the cross-section of said waveguide.

15. A beam traveling wave electron tube comprising in combination an evacuated envelope, an electron gun disposed at an end of said envelope for generating an electron beam of predetermined average electron velocity, a collector electrode positioned within the opposed end of said envelope, means .for directing said generated electron beam along a path axially throngs said envelope and impinging said beam upon said collector electrode, a helical waveguide of substantially rectangular crosssection surrounding said path and having one of its conductive bounding surfaces provided with a gap running lengthwise thereof and following the helical convolutions of said surface, said gap being disposed substantially centrally of said one bounding surface and per- .mitting energy radiation from said waveguide into the region of said electron beam, and means for introducing energy to and extracting energy from said waveguide, the axial dimension of said .gap being less than the axial dimension of the cross-section of said waveguide.

16. A beam traveling wave electron tube comprising in combination an evacuated envelope, an electron gun disposed at an end of said envelope for generating an electron beam of predetermined average electron velocity, a collector electrode positioned within the opposed end of said envelope, means for directing said generated electron beam along a path axially through said guide, said Waveguide being I r envelope and impinging said beam upon said collector electrode, a helical waveguide of substantiallyrectangular cross-section surrounding said path and having one of its conductive bounding surfaces provided with a gap running lengthwise thereof and following the helical convolutions of said surface, said gap being disposed substantially centrally of said one bounding surface and permitting energy radiation from said Waveguide into the region of said electron beam, and means for introducing energy to and extracting energy frorrisaid waveadapted to permit density modulation of said electron electric wave energy flowing therein, and to extract energy from a density modulated electron beam, the axial dimension of said gap being less than the axial dimen sion of the cross-section of said wave guide.

17. An electron tube comprising in combination a first flat helical conductive strip, a second helically formed conductive strip of pitch equal to that of said first strip andformed in 'edgewise contacting relation therewith, a helical gap formed between turns of said first strip being disposed substantially centrally of adjacent turns of said second strip, a cylindrical conductive enclosure for said second'stn'p, said strips and said enclosure defining a helical wave guide of substantially rectangular cross-section, means for"directing an electron beam axially of said helical wave guide, and spaced energy couplers associated with said wave guide.

18. A beam traveling Wave electron tube comprising i in combination an evacuated envelope, a helical cylinder,

an electron gun disposed at an end of said envelope for generating an electron beam of predetermined average electron velocity, a collector electrode positioned Within the opposed end of said envelope, means for directing said generated electron beam axially through said cylinder and impinging said beam upon said collector elec trode, a helical 'wave guide formed of channel shaped conductor having substantially rectangular cross-section, the open side of said channel shaped conductor engaging the outer cylindrical surface of saidhelical cylinder, means for introducing microwave energy to said helical wave guide for establishing a traveling wave therein, said helical wave guide permitting the inter-action of microwave energy flowing therein with said electron beam, and means for extracting energy from said helical wave guide.

19. ,A beam traveling wave electron tube comprising in combination an evacuated envelope, a helical cylinder, an electron gun disposed at an end of said envelope for generating an electron beam of predetermined average electron velocity, a collector electrode positioned within the opposed end of said envelope, means for directing said generated electron beam axially through said cylinder and impinging said beam upon, said collector electrode, a helical wave guide of substantially uniform pitch equal to the pitch of said helical cylinder, said helical wave guide being formed of channel shaped conductor of substantiallyrectangular cross-section and being coaxial vvith said cylinder, the open side of said channel engaging the cylindrical outer surface of said helical cylinder, means for introducing microwave energy to said helical Wave guide for establishing a traveling electromagnetic wave therein, the aforesaid pitch of said helical ,wave guide being arranged whereby the component 'of propagation velocity of traveling wave energy therein parallel'to said electron beam is substantially equal to said average electron velocity.

20. A beam traveling wave electrontube comprising in combination an evacuated envelope, a helical cylinder, an electron gun disposed at an end of said envelope for generating and directing an electron beam of predetermined average electron velocity through said cylinder,'a collector electrode positioned within the opposed end of said envelope, means for directing said' generated electron beam axially through said cylinder and impinging said beam upon said collector electrode, a helical wave beam in accordance with' establishing a traveling electromagnetic wave therein,

the aforesaid pitch of said'helical ave guide being arranged whereby the component of propagation velocity of traveling wave energy therein parallel to said electron beam is substantially equal to said'average electron velocity, means disposed adjacent said collector electrode adapted to extract energy from said helical wave guide at e an intensity level greater than said introduced energy.

21. A beam traveling wave electron tube comprising in' combination an evacuated envelope, a helical strip, an electron gun at one end of said envelope for generating an electron beam of predetermined average electron velocity, an axially opposed collector electrode at the opposite end of said envelope, said electron beam being directed axially along a path through said helical strip and toward said collector electrode, a helical wave guide of the same pitch as said strip, said helical wave guide being formed of channel shaped conductor of rectangular crosssection and being coaxial with said path, the open side of said helical wave guide channel defining a cylindrical surface contacting said helical strip and enclosing said electron beam path, means forintroducing microwave energy to said helical wave guide for establishing a travel- 7 ing electromagnetic wave therein, the aforesaid pitch of said helical Wave guide being arranged whereby the component of propagation velocity of traveling wave energy therein parallel to said electron beam is substantially equal'to said average electron velocity.

22. A beam traveling wave electron tube comprising in combintaion an evacuated envelope, means disposed at an end of said envelope for generating an electron beam of predetermined average electron velocity, a collector electrode positioned at the opposedend of said envelope, means for directing said electron beam along a path axially through said envelope and impinging said beam upon said collector electrode, a helically formed conductive strip in edgewise relation with said path, a substantially cylindrical conductive enclosure for said helical strip, said strip and enclosure defining a helical passage of substantially rectangular cross-section about said beam path, and means for introducing high frequency signal energy to said helical passage and establishing a traveling electromagnetic wave therein, and means axially opposed said energy introducing means for extracting energy from said helical passage.

23. A beam traveling wave electron tube comprising in combination an evacuated envelope, means disposed at an end of said envelope for generating an electron beam of predetermined average electron velocity, a collector electrode positioned at the opposed end of said envelope, means for directing said electron beam along a path axially through said envelope and impinging said beam upon said collector electrode, a helically formed conductive strip in edgewise and coaxial relation with said beam path disposed between said electron beam generating means and said collector electrode, a substantially cylindrical conductive enclosure for said helical strip, said strip and said conductive enclosure defining a helical passage of substantially rectangular cross-section about said path, means for introducing high frequency signal energy to said helical passage thereby establishing a traveling electromagnetic wave therein, and means for extracting signal energy from said helical passage, the pitch of said helical strip being arranged whereby the axial component of velocity of Wave propagation in said helical passage is substantially equal to said predetermined average electron velocity.

24. A beam traveling Wave electron tube comprising in combination an evacuated envelope, means disposed at an end of said envelope for generating an electron beam of predetermined average electron velocity, a collector electrode positioned at the opposed end of said envelope, means for directing said electron along a path beam axially through said envelope and impinging said beam upon said collector electrode, a first flat helical conductive strip surrounding said path, a second helically formed conductive strip of pitch equal to that of said first strip and formed in edgewise contacting relation therewith, a helical gap formed between turns of said first strip being disposed centrally of adjacent turns of said second strip, a cylindrical conductive enclosure for said second strip, said strips and said enclosure defining a helical passage of substantially rectangular cross-section about said path and communicating therewith through said helical gap, and means for introducing signal energy to said helical passage.

25. In combination, an evacuated envelope, electrode means within said envelope providing a stream of harged particles, a slow Wave transmission path in coupling proximity thereto, said slow Wave transmission path comprising a substantially cylindrical elongated conducting body having a' partially open conducting helical transmission channel on a curved surface thereof, with the open side of said transmission channel facing said stream.

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